FIG. 3 illustrates a diagrammatic cross section through a prior art surface-mountable outer contact of a semiconductor device of this type. With the aid of this surface-mountable outer contact 1, the semiconductor device is mounted on a higher-level printed circuit board 18. The higher-level printed circuit board 18 has contact connection surfaces 19, which are mounted on the top side 20 of the printed circuit board 18 and in terms of arrangement and size correspond to the arrangement and size of the outer contacts 1 of the surface-mountable semiconductor device 3. The top side 20 of the printed circuit board 18 may have a soldering stop resist layer 17, which only leaves clear a region, in which the outer contact 1 is to be arranged, at the corresponding contact connection surface. The outer contact on the underside 2 of the semiconductor device 3 is also arranged in a very similar way, except that in this case the outer contact connection surface 4 comprises two metal layers 9 and 10, the metal layer 10 being particularly suitable for forming a compound with the solder material of the outer contact 1. Outer contacts 1 of this type have a lead-containing solder material.
However, if the outer contact 1 consists of a lead-free solder material, the problem arises that intermetallic phases 16 are formed in the material of the outer contact 1 in a zone 21, with the result that the material of the outer contact 1 is greatly embrittled. When the outer contacts 1 are being secured to the underside 2 of the semiconductor device 3, as illustrated in FIG. 3, this brittle region is exposed, without any support, to the full stresses caused by shear forces, which occur on account of the differences in the coefficients of thermal expansion of the board part of the printed circuit board 18 and of the semiconductor 3. If these shear stresses get out of hand, there is a risk of the outer contact 1 tearing off from the semiconductor device 3 in the brittle material zone 21.
Furthermore, another surface-mountable outer contact is known from document U.S. Pat. No. 5,844,782. In this case, the external outer contact of the semiconductor device is arranged on an outer contact surface, the areal extent of which is smaller than the contact surface by which the outer contact is fixed to the underside of the semiconductor device. Therefore, the outer contact covers not only the outer contact surface but also the edge sides of the metal layer which has been patterned to form an outer contact surface. In principle, the outer contact surface projects into the solder bead of the external outer contact.
A similar structure for a surface-mountable outer contact is known from document U.S. Pat. No. 5,943,597; in this case, first of all a base of conductive material of smaller area extent is additionally deposited on the outer contact surface, and then a soldering ball which completely surrounds the base is applied to the base, so that the base deposited by electroplating projects into the soldering ball. The known surface-mountable outer contacts have the drawback that the critical transition zone from the outer contact surface or the base deposited by electroplating to the outer contacts referred to above is not supported in any way, and consequently there is a risk that in this relatively brittle transition zone the risk of tearing will increase still further in the event of corresponding thermal stresses and sheer stresses on the outer contact.
Patent application DE 103 52 349 discloses a semiconductor chip with flip chip contacts, and a process for producing it. The flip chip contacts are arranged on contact surfaces which are surrounded by a passivation layer covering the active top side while leaving clear the contact surfaces. For its part, this passivation layer has thickened portions which surround the contact surfaces and have an edge beading of polymeric passivation material which limits the extent to which the solder material of a solder ball can spread over the center region of the contact surface. This soft edge beading, consisting of polymeric passivation material, of the outer contact surface cannot mechanically support the critical and brittle transition zone from the solder ball to the outer contact surface.
However, the demands imposed on surface-mountable outer contacts and their reliability are becoming greater and greater, especially since ball grid array packages (BGAPs) with surface-mountable outer contact surfaces are nowadays being used as soldering contact points for a wide range of applications. Mobile appliances are becoming increasingly important in this context. Manufacturers of these appliances, in addition to the reliability tests that have hitherto being standard, are increasingly requiring evidence of what is known as mechanical shock resistance, also referred to as the drop test. In this context, it has been established that the drop test results of lead-free soldered joins are worse than those of lead-containing solders. The outer contacts break at the thinnest point in the soldered join, namely at the point with the greatest stress.
In the case of lead-free solder, this point is also distinguished by the fact that a brittle intermetallic phase of zone within the outer contact is formed close to the outer contact surface and is unable to withstand the shock-like mechanical stresses. The above solutions increase the risk of fracture in solder balls made from lead-free solders still further, since the outer contact surfaces or a pedestal or base which has been deposited by electroplating projects into the solder ball.
Other forms of solutions have proven either too expensive or not practicable. The especially expensive methods include introducing an underfill material between the outer contact surface and a higher-level circuit substrate on which the semiconductor device is to be surface-mountable, with this underfill material being intended to support the outer contacts in order to ensure that they are not torn off the semiconductor chip. In this case, however, an additional process, which involves using special tools to press an underfill material between the underside of the semiconductor chip and the top side of the higher-level printed circuit board, is required for the surface mounting of the semiconductor devices on a higher-level printed circuit board. An additional problem is the limited and restricted storage time of underfill materials of this type and the often greatly restricted access options for introducing the underfill material on the higher-level printed circuit boards and in the intermediate spaces.
In the case of other surface-mountable semiconductor devices, it is attempted to avoid the drawback by drastically reducing the size of the solder bead, which is associated with a reduction in weight and stress. However, this solution proves to be of only very limited use for industrial manufacture using the ball grid array package process that needs to be employed. Another way of avoiding the difficulties of the outer contacts becoming brittle in a boundary zone consists in using elastic solder beads which in the core consist of plastic and only the shell of which is composed of solderable material. However, elastic beads of this type have not to date found an application suitable for industrial manufacture. Moreover, the options for exchanging and replacing semiconductor devices when using surface mounting with elastic solder beads are extremely limited.
Further developments provide for the formation of what are known as non solder mask defined pads” (NSMDPs), which are intended to reduce the mechanical stress over a relatively large contact surface. The drawback of these NSMDPs of outer contact surfaces is the relatively small change in stress which these outer contacts will tolerate without being damaged. Finally, tests have also been carried out using what are known as “softer” alloys with correspondingly softer intermetallic phases at the boundary layer or boundary zone between outer contact surfaces and outer contacts. However, these tests have not to date managed to achieve a certain standardization of the alloys.
Finally, it is attempted, as mentioned in application DE 103 52 349, to support the at-risk location by means of what is known as a collar reinforcement made from a polymer. However, this solution has the drawback that even relatively minor changes in stress can lead to the supporting function being lost. The proposed solutions listed above are therefore in some cases inadequate for the shock stresses mentioned above, or alternatively too expensive to realize on an industrial scale.
For these and other reasons, there is a need for the present invention.